1. Field of the Invention
The present invention relates to a gripping type hand.
2. Description of the Related Art
In an operation of a robot to handle a workpiece or tool, it is a known configuration that a hand provided with a plurality of (generally about two or three) fingers is attached to the front end of an arm and the fingers are operated to open or close by a pneumatic pressure so as to grip the workpiece or tool. In this configuration, a so-called servo-hand which uses a servomotor as a drive source of the fingers and converts a rotary output of the servomotor to a linear motion through a ball-screw mechanism so as to open or close the fingers has been commercially developed. Further, while mostly in a research stage, a multi-finger hand which includes a plurality of (in general three or more) finger mechanisms provided respectively with joints, and which operates the finger mechanisms in various ways to grip an object, has been proposed as a hand mimicking a human hand.
For example, Japanese Unexamined Patent Publication (Kokai) No. 10-100089 (JP-A-10-100089) discloses a multi-finger hand having a plurality of multi-articulated fingers, each of which is provided with a force sensor. This multi-finger hand has a configuration such that, when gripping an object, the fingers are operated while monitoring external forces acting on the respective fingers by force sensors, the operations of the fingers are stopped at an instant when it is detected that the respective fingers contact the object, and a gripping force is generated in this state to grip the object. JP-A-10-100089 also describes that the gripping force of the hand is controlled so as to correspond to the external forces detected by the force sensors.
In the above conventional hand having a configuration operating the fingers by a pneumatic pressure, the gripping force depends on the cross-sectional area of a pneumatic cylinder and a working air pressure. The pneumatic cylinder usually operates so as to generate a constant air pressure, so that it is necessary to select the cylinder having optimal dimensions in accordance with the weight of an object to be gripped (i.e., a gripped object). Further, the opening/closing distance of the finger obtained by the cylinder is not so large, so that the size of the object capable of being gripped substantially depends on the combination of the cylinder and the finger. In other words, in order to grip objects having different sizes, it is necessary to suitably exchange the hand to another one including a cylinder and a finger corresponding to the size of the object. Therefore, in the case of a robot system using a single robot to handle a plurality of workpieces, the frequent exchange of hands is predicted, and thus an increase in cost in the system, an increase in a cycle time, an increase in a hand storage area, etc., may occur.
On the other hand, the above conventional servo-hand enables the fingers to have a large opening/closing distance, so that the range of dimensions of objects able to be gripped is increased. Further, by a torque control of the servomotor, it is possible to easily adjust the gripping force. However, for example, with a servo-hand having three fingers, the fingers are generally arranged at regular intervals (i.e., at every 120 degrees of center angles), so that it may be difficult for three fingers to grip the object when the gripping positions on the object are specified or due to the specified shape of the object. In this case, it is necessary to provide hands with different number or arrangements of fingers. Further, this type of conventional hand has no means for detecting that, for example, an external force causes an object to slip with respect to the fingers during a period when the hand grips the object. Therefore, for example, in a work for fitting the gripped object with another object by using a hand, a smooth fitting operation may become difficult.
Moreover, the conventional multi-finger hand, such as the above-described multi-finger hand of JP-A-10-100089, usually aims to improve the degree of freedom of the fingers, and, as a result, the structure thereof tends to become complicated and the operational control tends to become troublesome. Further, as described in JP-A-10-100089, in the system detecting a load acting on the hand by a force sensor to control a gripping force, a multi-axial force sensor unit, such as a six-axial force sensor able to detect a load by dividing the load into a total of six components of three-dimensional force and three-dimensional moment, a three-axial force sensor able to detect a load by dividing it into three-dimensional forces, and so on, is generally used. In general, the multi-axial force sensor unit detects three to six components of a load by a relatively small structure, so that it has a complicated, delicate mechanism, and that, for a stable and high-precision measurement, it is difficult to produce, calibrate and handle the unit. Therefore, it is required for a stable and high-precision control of the gripping force to pay the very careful attention to handle the multi-axial force sensor unit. If the multi-axial force sensor unit declines in performance or malfunctions, the expected control of the gripping force becomes difficult. Further, the multi-axial force sensor unit is generally expensive and may cause an increase in the cost of production of the hand. In this connection, it should be noted that the term “load” used in this application means the force and moment applied from the outside.